1. Field of the Invention
The present invention generally relates to a resonant inverter circuit, and more particularly to a resonant inverter or ballast.
2. Description of the Related Art
Fluorescent lamps are the most popular light sources in our daily lives. Improving the efficiency of fluorescent lamps significantly saves energy. Therefore, in recent development, how to improve the efficiency and save the power for the ballast of the fluorescent lamp is the major concern.
FIG. 1 shows a conventional inverter circuit with a resonant inverter circuit connected in series for an electronic ballast. Two switches 10 and 15 form a half-bridge inverter. The two switches 10 and 15 are complementarily switched on and off with 50% duty cycle at the desired switching frequency. An inductor 75 and a capacitor 70 form a resonant circuit to operate a fluorescent lamp 50. The fluorescent lamp 50 is connected in parallel with a capacitor 55. The capacitor 55 is operated as a start-up circuit. Once the lamp has been started up, the switching frequency is controlled to produce the required lamp voltage. A controller 5 is utilized to generate switching signals S1 and S2 to drive switches 10 and 15 respectively. The switch 10 is coupled to a high voltage source V+. The controller 5 is thus required to include a high-side switch driver to turn on/off the switch 10, which increases the cost of the circuit. Another drawback of this circuit is high switching loss on switches 10 and 20. The parasitic devices of the fluorescent lamp, such as the equivalent capacitance, etc., are varied in response to the temperature variation and the age of the lamp. Besides, the inductance of the inductor 75 and the capacitance of the capacitor 70 are varied during mass production. The objective of the present invention is to provide a low cost inverter circuit that can automatically achieve soft switching for reducing the switching loss and improving the efficiency of the ballast.